Sports bras are designed to reduce breast movement during exercises, but previous studies on breast movement were based on various, unclear and unreliable reference systems. Heretofore, there is no standardised, valid and reliable method to evaluate relative three-dimensional (3D) breast movement affected by different design features of sports bras; and there is no literature to predict the 3D force acting on the breasts during activities. Both empirical and theoretical studies on this unexplored area are needed to provide a scientific basis for the functional design of supportive sports bras. The aim of this study was therefore to establish a reliable method to evaluate 3D breast movement and to determine the effective design features of supportive sports bras. The specific objectives were to derive and validate a new Breast Coordinate System (BCS) for investigating 3D breast movement, so as to identify the most effective bra features and to analyse the effects of breast volume and bra strap properties on breast movement, then to develop theoretical models of breast force generated during jogging in braless and sports bra conditions. To achieve the objectives, the research framework contains five principal parts, including (i) deriving a new BCS; (ii) evaluating the 3D breast displacement and trajectories of six breast points on four subjects; (iii) identifying the effective bra features of seven different sports bras; (iv) analysing the effects of 11 breast volumes and 10 different shoulder straps on breast displacement; and (v) building theoretical models of 3D breast force.A new BCS was derived and validated based on a reliable reference on the thoracic cage and a local breast origin. The relative 3D breast displacement and trajectories were evaluated and compared among different breast points, bras, and activities. Paired t-tests showed that the displacement at the nipple was significantly different from other breast points. The relative 3D nipple displacement during jogging was reasonably smaller than previous results for D-cup women. For the subjects in this study, the most effective bra features were identified. Shoulder strap elongation was significantly correlated with the Reduction percentage of Breast Displacement (RBD). The vertical breast movement significantly increased with breast volume. 3D mechanical models have been developed based on a system comprising a mass, springs and dampers. The orthogonal force exerted on the breasts during jogging with or without a sports bra was derived. Breast mechanical models with different shoulder straps predicted that wider shoulder straps were more effective in supporting the breast weight, and the force acting on the shoulder straps was related to the shoulder straps' styles and the positions on the shoulders. This research has developed a new, valid and reliable breast coordinate system and the techniques to evaluate the effectiveness of sports bra in terms of comprehensive 3D relative breast displacement for different breast volumes. The breast mechanical models were developed to estimate the 3D breast force in different wearing conditions. The new methods will contribute to future research on human locomotion and the design of sports bras.

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